Major Change in Northwest Weather: Rain Moves in, Smoke Moves Out

A strong upper level trough will move inland over the region tonight and tomorrow morning bringing cooling weather, clouds, some precipitation, and the removable of the smoke layer aloft.

To start,  here are the upper-level (500-hPa, about 18,000 ft) charts for the next few days.

This morning  (5 AM) shows a big ridge over the western U.S., but a strong closed low/trough offshore.

Tomorrow morning at 5 AM, the trough/low is moving into our coast.

The trough moves through on Saturday and then moves northeastward into Canada on Sunday (11 PM Sunday is shown below)

The initial approach of the trough is associated with enhanced southerly flow that brought up smoke from California, but as it passes, the winds will turn more westerly, driving the smoke to the east.  This progression is shown by the HRRR-SMOKE model output. This morning at 10 AM, there was fairly dense smoke aloft (see figure), but far less at the surface.

But by 2 AM, western Washington is clear, as the densest smoke moves over eastern WA and Idaho.

 Precipitation should start moving in by 5 AM, illustrated by the UW WRF model precipitation for the 3-h period ending that time.

 Heavier rain is predicted during the subsequent three hours, particularly along the coast, the Olympics, and SW Washington.

The showers move to the Cascades during the afternoon.

  And continue while slowly declining during Saturday evening.

Seattle will not got a lot from this event and we do expect some embedded thunderstorms.   Sunday should be dry and warmer (around 80F).

Finally, it is interesting to look back at the temperatures at Seattle Tacoma Airport during the last 12 weeks compared to the normal highs (purple) and normal lows (cyan).  We have had a series of heat waves during the past two months, with some intervening cool periods.

Clearly, this summer was warmer than normal, something shown graphically with the following map of the temperature differences from normal over the past 60 days.  Western WA has been 1-3F above normal, with California being even warmer (perhaps 3-4F above normal for the period).